1. Field of the Invention
The present invention relates to a light emitting device, and more particularly relates to a light emitting device including a semiconductor light emitting element and phosphors for emitting a light having a different wavelength from the light emitted from the semiconductor light emitting element.
2. Description of the Related Art
Light source devices or light emitting devices in which a semiconductor light emitting element and a fluorescent element are combined (refer to JP-A 2005-205195 (KOKAI), JP-A H07-282609 (KOKAI), and JP-A 2006-210887 (KOKAI)) are known. The fluorescent element absorbs an excitation light emitted from the semiconductor light emitting element and emits a light having a different wavelength from the excitation light.
In JP-A 2005-205195, a light emitting device having a high luminance and a high color rendering characteristic is described. In the light emitting device, an optical fiber is used as a waveguide for an excitation light and a fluorescent element is attached to the tip of the optical fiber.
Also, in JP-A H07-282609, a durable and safe light emitting device having low electric power consumption is proposed. Further, the light emitting device can provide a very suitable illumination light, such as a white light, having a sufficient output power. The light emitting device includes a semiconductor laser diode emitting a laser light, a lens diffusing the laser light from the semiconductor laser, and a fluorescent element converting the laser light to a visible light.
Moreover, in JP-A 2006-210887, a light emitting device, which includes a semiconductor light emitting device emitting an excitation light for phosphors and a dispersing element of the phosphors, having a high light extraction efficiency is proposed. In the light emitting device, a radiated visible light from the phosphors of the dispersing element is extracted from the incident side of the excitation light in the dispersing element.
However, in the earlier light emitting devices described in the foregoing patent documents, the following points have not been considered. In the light emitting device of JP-A 2005-205195, the emitting direction of the light from the light emitting element and the radiating direction of the light from the fluorescent element are identical. Even in the light emitting device of JP-A H07-282609, the output direction of the laser light of the semiconductor laser diode, the diffusion direction of the light of the diffusing lens, and the radiating direction of the light from the fluorescent element are identical. Also, even in JP-A 2006-210887, the light emitting device has a structure in which a part of the excitation lights scattered by the phosphors of the dispersing element may be emitted from the incident side of the excitation light with the visible light. Thus, it is insufficient to suppress the short wavelength light scattered by the phosphors of the dispersing element.
Accordingly, due to the leakage of the excitation light emitted from a light emitting diode or a semiconductor laser diode, it is not desirable to use the light emitting element which radiates a high energy light, such as an ultraviolet light and a laser light, because it is dangerous to view directly. Also, in order to decrease the leakage of the excitation light in such structures, it is necessary to decrease the output power of the semiconductor light emitting element, or to increase the thickness of the fluorescent element. As a result, the efficiency of the light emitting device decreases.
Also, it is required that the light emitting device shall maintain the output power of the semiconductor light emitting element constant. However, when the output power of the light emitting element is changed, the excitation magnitude of the phosphors may be changed. Also, spectrum of the extracted light intensity with regards to the wavelength may be changed. Therefore, there is a problem that the color tone of the light obtained from the light emitting device is changed. Thus, in the earlier light emitting devices, it is substantially difficult to adjust the quantity of the light.
Moreover, it is not desirable to use the light emitting element radiating a high energy light for a light emitting device. Thus, it is difficult to apply the light emitting device to a lighting apparatus, an image display and the like, in which a high luminance is required.